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When we talk about the substance $\ce{H2O}$ with a molar mass of $\pu{18.0 g/mol}$, what hydrogen and oxygen isotopes are present in the $\ce{H2O}$ molecules? In other words, what isotopes do the symbols represent?

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Most periodic tables quote the molar mass of an element as the average mass of the element given the natural abundance of the isotopes. Chlorine, for example, has a standard atomic weight of 35.45 which is a weighted average of ~75% 35Cl and ~25% 37Cl, the two common isotopes.

But isotopes of hydrogen and oxygen are rare. So the vast majority of water molecules will not contain them. So, at least to one decimal place, most water will have a molar mass of 18.0.

At it's natural abundance of about 0.25% 18O won't noticeably affect the molecular weight of water until you can measure it to at least 3 decimal places of precision. Slightly simplifying (assuming pure isotopic masses are exact integers, which they are not), water containing that isotope at its natural abundance will have an an average mass of 18.005 rather than 18.000.

When working with normal (unenriched) substances the best assumption is that they will contain the normal isotopic abundance of the component elements (which are well known and, for most elements, don't very much by source). But the effect on molecular mass will often be small and you will need a fair amount of precision to notice it. If you are not working to very precise masses, it is fair to assume natural isotopic abundance.

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It's impossible to find isotopic composition of a compound with a molecular weight containing only one decimal point ($\pu{18.\underline{0} g mol-1}$). Typically, 6th decimal place is required in order to refine percentage of each isotope.

Symbols themselves (such as $\ce{H}$ and $\ce{O}$) don't carry any additional information except atomic number $Z$. For isotopes you need to use so-called AZE notation: $^A_Z\ce{E}$, for instance $\ce{^1_1H}$ and $\ce{^{17}_8O}$. Since $Z$ is already known from $\ce{E}$, simplified notation is often used instead: $\ce{^1H}$ and $\ce{^{17}O}$.

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  • $\begingroup$ I'm just confused as to where the molar mass of a substance comes from. As in how do we know how much a mole of a substance weighs exactly? I could have 2 samples of 1 mole of water, and one could weigh more than the other because one may contain more heavy water (D2O) than the other. $\endgroup$ – user70490 Jan 3 at 11:00
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    $\begingroup$ Unless you took one water sample from the nuclear reactor and the other one from the river, isotopic distribution is going to be the same as elements have uniform isotopic distribution in nature. You probably want to read these topics: chemistry.stackexchange.com/questions/71942/… and chemistry.stackexchange.com/questions/38082/… and any introductory chemistry textbook. $\endgroup$ – andselisk Jan 3 at 11:06
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Hydrogen has 2 stable isotopes,H-1 and H-2, Oxygen has 3 stable isotopes H-16, H-17 and H-18. There is very little of the minor isotopes so the molecular weight of water is 18.

However hydrogen and oxygen are light elements and differences in atomic weights have detectable differences in physical and chemical properties. For example H-2 compounds can have lower reaction rates than the H-1. Boiling points and freezing points of various isotopic forms of water differ slightly. Indeed it possible to distil water to separate the various isotopes.

This also happens in natural systems where minute variation in isotopic contents trapped in ancient ice can be used to gauge the ocean temperature of that time

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